This invention relates to systems for electronically monitoring the condition of a tractor-trailer and, in particular, to a monitor contained in each tire on a tractor-trailer.
Truck tires, unlike passenger car tires, are designed for high mileage, e.g. 200,000 miles per tire. There are some trade-offs in such a design. One trade-off is that a truck tire has a much harsher ride than a tire for a passenger car. Another trade-off is that a truck tire has a very narrow range of optimum pressure and the optimum pressure changes with load (an empty trailer should have tires at a lower pressure than a full trailer). Re-tread tires commonly used in the trucking industry have a particularly narrow range of optimum pressure.
There are known tire monitoring systems for tractors, trailers, and construction vehicles use dedicated electronics to identify each tire. For example, U.S. Pat. No. 5,109,213 (Williams) discloses a system in which a small transmitter is attached to the outside of a wheel. A DIP (dual in-line package) switch in each transmitter permits one to identify each wheel uniquely. If a wheel is replaced or if tractors and trailers are mixed and recombined, e.g. in double trailer units, then the monitoring system will not function correctly unless each wheel is re-coded. Each wheel's identification code must be loaded manually by way of the DIP switch, a tedious task. Manually setting codes is highly undesirable because it increases the down-time of a trailer and increases the cost of using the trailer.
U.S. Pat. No. 4,978,941 (Brown) discloses a tire monitoring system in which a receiver is located near each tandem axle on a tractor-trailer for detecting low power transmissions from transmitters in each wheel hub. Each wheel is not uniquely identified but the choice is narrowed by the proximity of a receiver to the suspect tires.
U.S. Pat. No. 4,742,857 (Gandhi) discloses a tire monitoring system in which a microprocessor tracks consecutive readings to determine trends and corrects tire pressure readings for temperature. U.S. Pat. No. 5,231,872 (Bowler et al.) also discloses a tire monitoring system in which tire temperature and tire pressure are measured. In the Bowler et al. system, all tires on a trailer have the same identification number which obliges the driver to inspect a group of tires to determine the problem, if any.
Tire monitors of the prior art use a small transmitter to communicate with a computer elsewhere in the vehicle. The electrical environment of a tractor-trailer is not conducive to effective communication. Not only are the wheels and chassis of a vehicle electrical ground, steel belts in the tires act as an electromagnetic shield to transmissions from the wheel. The electrical system of most vehicles is noisy due to the operation of a generator or an alternator and due to static discharges.
One can increase the power of the signal transmitted by a tire monitor but this significantly reduces the life of the battery powering the monitor. Known tire monitors are typically mounted on the outside of a wheel because the life of the battery in the monitor is shorter than the expected life of the tire. It is desired that the battery for a tire monitor have a life exceeding the life of the tire. It is also desired to improve the efficiency of a tire monitoring system by improving the RF coupling between the tire monitors and a receiver.
Radio transmissions from a tire monitor when a wheel is not in service also reduce the life of the battery. An on-off switch on a tire monitor can be set to the wrong position accidentally and is subject to mechanical failure due to vibration, impact, temperature extremes, or moisture. Installing a battery when a tire is put into service is not practical and is impossible when the monitor is inside a tire.
Tire monitors of the prior art that uniquely identify each tire typically transmit infrequently in order to conserve battery power and in order to avoid "collisions" with transmissions from monitors in other tires on a vehicle. This system works unless the condition of a tire deteriorates rapidly and the driver needs to know immediately that something is very wrong with a tire. There is a need for a tire monitor that can indicate an alarm condition and yet will last for the life of a tire if there is no damage to the tire.
In view of the foregoing, it is therefore an object of the invention to provide a tire monitor having a life equal to or greater than the expected life of a tire.
Another object of the invention is to provide a tire monitor that uniquely identifies the tire to which it is attached with a code furnished during manufacture of the tire monitor.
A further object of the invention is to provide a tire monitor that adapts to the condition of a tire by more closely monitoring the tire if the condition changes quickly.
Another object of the invention is to provide a tire monitoring system with improved coupling between the tires and a receiver.